In a diesel engine, an exhaust pipe is provided with a DPF in order to remove particulate matter (hereinafter referred to as PM) from exhaust gas from the diesel engine to purify the exhaust gas. A DPF is designed to collect PM in a honeycomb structure constituted by a porous ceramic. While excessive accumulation of collected PM impedes circulation of exhaust gas, the PM accumulated in the DPF can be removed through incineration by raising exhaust gas temperature. This is known as DPF regeneration.
Conventionally, in multistage injection in which an oxidation catalyst (hereinafter referred to as a DOC) made of platinum or the like is installed upstream of the DPF and fuel injection is performed in multiple stages in order to obtain engine thrust, methods of raising exhaust gas temperature to a level where PM can be incinerated include a method involving raising exhaust gas temperature to DOC activation temperature by increasing fuel injection amount, performing fuel injection for DPF regeneration (post-injection) at an appropriate subsequent crank angle, supplying hydrocarbon (hereinafter referred to as HC) to the DOC by the post-injection, and raising the exhaust gas temperature to PM incineration temperature using heat produced by oxidation of the HC.
However, performing post-injection causes oil dilution in which the injected fuel becomes mixed into an engine lubricant and dilutes the lubricant. In addition, mixing of unburnt fuel of the post-injection into an EGR device causes a decline in performance of an EGR cooler or a failure of a piston ring.
Therefore, conventionally, EGR control is suspended by fully closing an EGR valve during DPF regeneration when post-injection is performed.